This invention relates to magnetic transmission devices using the edge-guided mode of propagation, and, more particularly, to microstrip and stripline transmission devices such as isolators, multi-port circulators, reflection-type amplifiers, and phase shifters.
In microstrip and stripline transmission devices using ferrite dielectric slabs magnetized perpendicular to the ground plane, it is known that the fields are concentrated along one edge and decayed along the opposite edge, so that microwaves are said to be guided or propagated along the edge. Using the principles of such edge-guided mode, it has been proposed to provide ferrite transmission devices of nonreciprocal modes, such as isolators and phase shifters, by asymmetrically loading the edges with a Mylar film coated with thin-film metallization (M. E. Hines: "Reciprocal and Nonreciprocal Modes of Propagation in Ferrite Stripline and Microstrip Devices", IEEE Transactions on Microwave Theory and Techniques, vol. MTT-19, No. 5, May 1971). For a better understanding of the ferrite transmission device using the principles of edge-guided mode and the device proposed by M. E. Hines, we now refer to FIG. 1a through FIG. 1d, in which:
FIG. 1a is a schematic perspective view showing a microstripline transmission device,
FIGS. 1b and 1c are graphs showing the distribution of microwaves in the transverse direction of a conductor, wherein FIGS. 1b and 1c show distribution of the waves travelling in the forward direction A and reverse direction B, respectively, and
FIG. 1d is a schematic perspective view showing an isolator proposed by M. E. Hines.
Referring to a microstripline transmission device shown in FIG. 1a in which a conductor plate member 1 is mounted on a ferrite slab 2 placed on a ground plane 3, when a DC magnetic field is applied perpendicular to the ground plane 3 as shown by an arrow Hdc, microwaves travelling in the forward direction of an arrow A are concentrated along the left side edge of the conductor plate member 1 and decay along the right side edge thereof as shown in FIG. 1b, while microwaves travelling in the reverse direction shown by an arrow B are concentrated along the right side edge of the conductor plate member 1 and decay along the left side edge thereof as shown in FIG. 1c.
On the basis of the principles of a edge-guided mode of propagation, it has been proposed that when a lossy material, such as capacitance loading material or a resistive sheet (e.g. mylar) coated with carbon powder, 4 is provided along one edge of the conductor, for example along the right side edge as shown in FIG. 1d, microwaves travelling in the forward direction shown by an arrow A propagate along the left side edge, while microwaves travelling in the reverse direction shown by an arrow B are mostly absorbed in the lossy material 4. Accordingly, it is said that such ferrite transmission device can be used as an isolator.
However, in such an isolator, although microwaves inserted in the forward direction are propagated along one edge opposite to the lossy material, it has been noted that a relatively large amount of microwaves in the reverse direction are also propagated without being absorbed in the lossy material, so that the forward-reverse ratio is relatively low. For example, in microwaves having frequencies of 5.0 - 7.5 GHz, the insertion loss is about -1dB, isolation is about -20dB, and thereby the forward-reverse ratio is 20 at most.